In microwave design and in other fields of electronic engineering, thin film capacitors have been utilized for quite some time. There are three major problems, however, that frequently confront microwave design engineers in the use of thin film capacitors. Thin film capacitors generally have a polar characteristic that is typically manifested by eratic and inefficient capacitor operation and by unequal breakdown voltage, depending upon the direction of capacitor bias.
The other problems most frequently encountered in thin film capacitor design are sensitivity of the capacitor structures to elevated temperature and questionable reliability history. Thin film capacitors are sensitive to elevated temperatures because the various constituents from which the capacitors are composed typically have materially different coefficients of thermal expansion. For example, where aluminium is utilized as one of the film electrode layers of a thin film capacitor structure, the various structural layers of the capacitor will be stressed substantially at elevated temperatures because of the thermal mismatch between the aluminum and the other components of the capacitor. Thermal mismatch will cause stressing of the films and therefore can cause the leakage currents of the capacitor to be grossly increased an in some cases fracturing or distortion of the dielectric layer will cause the capacitors to become shorted. Linear expansion of the aluminum film is typically substantially greater than the linear expansion of a dielectric film to which the aluminum film may be adhered and, therefore, distortion or disruption of the dielectric film may occur as the capacitor is heated either during normal circuit operation or during assembly operations.
Questionable reliability of thin film capacitors results from stressing of the dielectric that results from mismatch of the linear thermal expansion coefficient. Other matters affecting reliability of thin film capacitors include the effect of moisture on the capacitor structures and the effect of heat including heat of circuit operation and the heat that is generated during assembly of the circuits to other components.
A currently available thin film capacitor structure in common usage utilizes anodized tantalum oxide (Ta.sub.2 O.sub.5) as the dielectric film under which a film of aluminum is utilized to minimize the series resistance of the device. The series resistance of a capacitor is particularly important for signal bypassing at microwave frequencies.
Utilization of a thin aluminum film over which anodized tantalum oxide is deposited (a thin film capacitor structure that is typical) can cause serious problems from the standpoint of heat. Two factors are detrimental to aluminum-tantalum oxide thin film capacitors at elevated temperatures, such as temperatures in the range of 200.degree. C. Aluminum has a substantially different linear coefficient of thermal expansion as compared to other thin film layers in a thin film capacitor structure. For example, the linear coefficient of thermal expansion of aluminum is .alpha.A1 = 20.times.10.sup.-.sup.6 /.degree.C. while the linear coefficient of thermal expansion for other layers of typical capacitor structures may be defined as .alpha..ltoreq.12.times.10.sup.-.sup.6 /.degree.C. It is obvious that a thermal mismatch occurs when two thin film layers of radically different coefficients of linear thermal expansion are adhered to one another. As is typically the case, the dielectric film interposed between two films of aluminum or in contact with an underlying aluminum foil will be substantially overstressed and distorted at elevated temperatures. When this occurs, the capacitor may become shorted if the thin dielectric film ruptures or the capacitor may be subject to erratic operation resulting from overstressing of the dielectric material.
Aluminum, the metal most often employed to form deposited electrode layers in thin film capacitors, has a low temperature of recrystallization. Recrystallization is observed in thin aluminum films at temperatures as low as 170.degree. C. Thus, above 170.degree. C. recrystallization would tend to take place. The distortion of the aluminum film that occurs during recrystallization would be aided by the stress that is induced by linear thermal expansion of the aluminum during heating and cooling thereof. While 170.degree. C. is rarely an operational temperature, such temperature is often reached during circuit fabrication and during assembly of various circuits. Recrystallization of aluminum causes great changes in the topology of the bottom electrode of thin film capacitors. The movement in the aluminum film strains and disrupts the relatively thin dielectric film and causes changes in the relationship of the dielectric film to the aluminum films laminated thereto that result in either grossly increased leakage currents or result in actually shorting the capacitors.
Investigations have determined that no high conductivity metal other than aluminum is known to be useful beneath anodized tantalum thin film capacitors. This is caused by the anodization process, because aluminum is the only high conductivity metal that is itself anodizable. Other metals having characteristics of high conductivity would be completely etched away or would cause the anodization process to be short circuited. This situation arises because the tantalum film is of such a thickness that pinholes are always present, and because some mixing of the tantalum and the underlay material occurs during deposition of the tantalum regardless of the manner by which the tantalum is deposited.
There are three significant approaches that may be taken in order to eliminate the problem caused by the aluminum underlay. It may be found desirable to eliminate the aluminum underlay and utilize only the conductivity provided by the thin film capacitors with a series resistance less than a few ohms. If the aluminum underlay is eliminated, the electrical performance of the thin film capacitor must be sacrificed for purposes of reliability. The resulting thin film capacitors would be quite reliable but the series resistance factors of such capacitors would be less than desirable.
A second alternative might be to employ some modified form of pure aluminum for an underlay material. Alloying other elements in very small percentages with the aluminum would provide some significant assistance by raising the recrystallization temperature of solid solution. Alloying would also increase the resistivity of the film, but it would still remain much more conductive than a thin film of tantalum. The elevated temperature thermal mismatch and consequent stress would still be present in such a structure, since alloying the aluminum would change its linear coefficient of expansion by only an insignificant degree. Thus, this approach could be utilized to increase reliability of thin film capacitors, but the change in high temperature reliability would be only moderate, thereby resulting in the production of capacitors that still present problems of reliability from the standpoint of temperature. If, during manufacture of electronic components, the temperature of such a capacitor is raised above the recrystallization temperature of the alloy, it is possible for the topology of the bottom electrode to be substantially changed, which obviously would result in significant changes in the operational capability of the capacitor structure.
A third possible approach to the problems caused by aluminum underlay is to abandon the use of anodized tantalum. If the dielectric layer is formed by a method other than anodization, then nearly any metal may be chosen for the bottom electrode. It is necessary, however, that the metal film underlays be highly conductive and, until the present time, no significant advance has been made in the utilization of metals other than high conductive metals for forming the electrodes of a thin film capacitor structure. It has not been considered practical to utilize a thin film deposit of a metal having rather poor conductive qualities because of the necessity for the series resistance of the capacitor to be low for radio frequency by-passing at microwave frequencies. Accordingly, it is a primary object of the present invention to provide a method of fabricating thin film capacitor structures by employment of a novel assembly of capacitor elements that does not include anodized tantalum.
It is further an object of the present invention to provide a novel process for fabrication of thin film capacitors wherein the thin film metal layer, the dielectric layer and substrate all have substantially the same coefficient of thermal expansion.
It is an even further object of the present invention to provide a novel method of fabricating thin film capacitors utilizing a connector overlay that enables such capacitors to be utilized as a component of a thin film circuit or in the alternative may be ultilized in beam lead assembly as part of a hybrid circuit.
Among the several objects of the present invention is noted the contemplation of a novel process for fabricating a thin film capacitor wherein a rather poor conductive material may be utilized for its high temperature of recrystallization without sacrificing the series resistance qualities that are necessary for signal bypassing at microwave frequencies.
It is also an important feature of the present invention to provide a novel method of fabricating a thin film capacitor structure that utilizes a counter electrode formed of molybdenum or tungsten, having a metal film deposited thereon which film may be of rather poor conductive characteristics, molybdenum and tungsten cooperating with the metal film to reduce resistive losses on either side of the dielectric material of the capacitor and preventing the operating frequency of the capacitor from being reduced.
Another object of the present invention notes contemplation of a novel thin film capacitor structure incorporating deposited metal films composed of a metal having a very high temperature or recrystallization thereby preventing changes in the topology of the electrodes of the capacitor by heat during circuit operation or during circuit assembly procedures.
It is another important object of the present invention to provide a novel method of fabricating thin film capacitors wherein all of the various laminar thin film layers of the capacitor are compatible from the standpoint of the linear coefficient of thermal expansion thereof.
The present invention also contemplates the provision of a novel thin film capacitor structure that may be fabricated according to the method set forth herein.
Also contemplated by the present invention is a novel thin film capacitor structure and method of fabricating the same which yields commercially producible thin film capacitors that are simple in nature, reliable in use and low in cost.
Other and further objects, advantages and features of the present invention will become apparent to one skilled in the art upon consideration of the present invention. The form of the invention, which will now be described in detail, illustrates, the general principles of the invention, but it is to be understood that this detailed description is not to be taken as limiting the scope of the present invention.